This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
Rocket motors used in big display fireworks usually belong to class IV
fireworks (as everything that is not BAM-approved, such as foreign motors)
and are limited to people with the appropriate license. Even worse:
if you stuff such a motor in a model rocket, it is automatically
considered display fireworks and requires a (costly) announcement to the
appropriate officials like any other display firework.